1. Field of the Invention
The present invention relates to a dual-focus switchable lens in a three-group structure, and more particularly to a dual-focus switchable lens, to be mounted on a mobile phone or the like, suited as a quite small-sized imaging lens having a wide angle-of-view and a magnification ratio between wide-angle and telephoto positions of 1.9 or greater.
2. Description of Background Art
Recently, a variety of small-sized lenses have been developed to be mounted on small-sized digital cameras, etc.
Above all, those described in JP-A-2004-93647 and JP-A-2005-84647 are known as imaging lenses, favorable in optical performance, each having a zoom function and a simple structure with three groups and six lenses.
Meanwhile, there is recently an increasing demand for an imaging lens to be mounted on a personal digital assistant, such as a cellular phone.
In the imaging lens for mounting on such a personal digital assistant, performance improvement in image quality, distortion, etc. is desired equally in degree to those mounted on the usual camera, such as the digital camera described in JP-A-2004-93647 and JP-A-2005-84647. Due to the requirement of size and thickness reductions for the personal digital assistant, there is a further increasing requirement for size-reducing the imaging lens to be mounted thereon. Meanwhile, there is a trend to provide a variable-magnification function for such an imaging lens to be mounted on a personal digital assistant.
However, where providing a variable-magnification function through utilization of optical zooming, two lens groups minimally must be moved for focal adjustment over the entire zoom range, thus complicating the structure and inevitably increasing the lens overall length.
Meanwhile, the three-grouped zoom lens, say described in JP-A-2004-93647 and JP-A-2005-84647, is structured compact. However, those are structured such that, by moving both the first and second lens groups during zooming, the imaging plane is suppressed from moving due to magnification change simultaneously with changing the magnification. For this reason, during zooming, the first lens group the closest in position to the object is moved back and forth. For the personal digital assistant including a cellular phone, it is not desirable to move the front, or first, lens group back and forth in respect of compactness, etc. Besides, there is a fear that dust, etc. possibly intrude in the lens system, which is a problem desirably to be avoided.